Designing for crummy profiles
In addition to monitor color (which I wrote about last week), the profile the monitor is set to can also be an obstacle to consistent display of web design. Look at the picture below:

Believe it or not, the text and background have the same RGB values in each example, but the profile chosen gives it a different appearance. The default OS profile, demonstrated in the last example, can give the most drastic difference, as it is in this example.
Unlike in printing, where the shop can be set to standard profiles and color consistency can be ensured, on the web you must always expect the worst. If I had used that text color in my design, someone stuck with the default profile may only see gray. Indeed, your monitor may be way off from the intended color - I suspect that my huey profiles are creating a red tint in my display, for example.
The best practice, again, is to avoid being subtle with your color.